The science of cheese
Cheesemaking is the controlled process of removing water from milk. This process concentrates the milk’s protein, fat and other nutrients and increases its shelf life. Cheesemaking is one of the earliest examples of biotechnology.
An ancient biotechnology
Cheesemaking is an ancient biotechnology that dates back to the domestication of animals. It is thought that it was probably first discovered by observing the accidental souring of milk and then pressing and salting the solid curd to preserve it for later consumption.
Get information sheet: Ancient biotechnology
Cheesemaking involves coagulating the casein protein in milk and then separating the milk into solid curds and liquid whey. The liquid whey is drained away, and the curds are salted, shaped and left to ripen in a controlled environment.
Microorganisms are used in each step of this process and determine the flavour and texture of the final cheese.
Bacteria acidify milk
Acidifying (souring) milk helps to separate the curds and whey and control the growth of undesirable bacteria in cheese. Usually special ‘starter’ bacteria are added to milk to start the cheesemaking process. These bacteria convert the lactose (milk sugar) to lactic acid and lower the milk’s pH.
There are two types of bacteria used for this process:
- Mesophilic bacteria thrive at room temperature but die at higher temperatures. They are used to make mellow cheeses, such as Cheddar, Gouda and Colby.
- Thermophilic bacteria thrive at higher temperatures, around 55 °C, and are used to make sharper cheeses such as Gruyère, Parmesan and Romano.
Enzymes speed up coagulation
Some cheeses are curdled only by acidity. For example, paneer cheese is made using lemon juice to curdle the milk and cottage cheese is made using mesophilic bacteria. However, for most cheeses, rennet is also added to the milk after a starter bacteria. Rennet is a mixture containing the active enzyme chymosin. Rennet speeds up the coagulation of casein and produces a stronger curd. It also allows curdling at a lower acidity, which is important for some types of cheese.
Casein proteins coagulate
Milk is about 86% water but also contains fat, carbohydrate (mainly lactose), proteins (casein and whey), minerals and vitamins. Milk is an emulsion of fat globules and a suspension of casein micelles. These are suspended in the liquid phase of milk that contains dissolved lactose, whey proteins and some minerals.
The chymosin in rennet breaks down the kappa casein on the surface of the micelles changing them from being hydrophilic to hydrophobic. This causes them to aggregate together, trapping fat and water molecules in the developing curd. Further processing of the curd helps remove more water and compress the curd to form a solid cheese.
Sources of rennet
Rennet comes from the stomachs of young mammals that have a diet of mostly milk. Rennet from calf stomachs was the major source of chymosin until the 1960s when it was predicted that increasing demand for meat and increasing cheese production would lead to a shortage of calves and therefore a shortage of rennet. This led to the development of substitutes including extracts from adult cows and pigs, fungi and, more recently, microorganisms that have been genetically modified (GM) to yield GM rennet. GM rennet is the most commonly used today because it is the most cost-effective to produce.
Releasing the whey
After separating curds and whey, further processing of the curds helps release more of the whey trapped in the network of micelles before it is drained away. The exact processing steps vary depending on the type of cheese. However, generally, the curds are captured, pressed and moulded to form blocks of cheese.
Historically, whey was considered a waste product of cheesemaking. However, growing concern over the environmental impact of its disposal encouraged research to better understand the properties and potential uses of whey. Increasing scientific understanding and technological advances have led to a wide range of uses for whey and established it as a valuable coproduct of the cheese industry.
Get information sheet: Uses of whey
Ripening the cheese
Cheese is left to ripen, or age, in a temperature and humidity-controlled environment for varying lengths of time depending on the cheese type. As cheese ripens, bacteria break down the proteins, altering the flavour and texture of the final cheese. The proteins first break into medium-sized pieces (peptides) and then into smaller pieces (amino acids). In turn, these can be broken down into various, highly flavoured molecules called amines. At each stage, more complex flavours are produced.
During ripening, some cheeses are inoculated with a fungus such as Penicillium. Inoculation can be either on the surface (for example, with Camembert and Brie) or internally (for example, with blue vein cheeses). During ripening, the fungi produce digestive enzymes, which break down large protein molecules in the cheese. This makes the cheese softer, runny and even blue.
Get information sheet: Creating different cheese characteristics
Transgenic cow research reveals potential to increase cheese yield
AgResearch’s transgenic cow research has produced transgenic cows with extra bovine kappa casein genes inserted in their genome. This resulted in increased kappa casein in their milk. Increased kappa casein levels are associated with better cheesemaking properties, higher cheese yield and higher calcium content.
Get information sheet: Transgenic cows making modified milk
- 11 April 2012